Plug memory ordering holes in buf_ring_enqueue. For at least some
users this patch eliminates the races previously discussed on the
mailing list.

Submitted by:	oleg
Reviewed by:	kmacy
MFC after:	2 weeks
Tested by:	kmacy,rpaulo


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273866 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f